This invention relates to electrophoretic display apparatus in general and more particularly to methods for fabricating such displays having a dual anode structure.
A dual anode, flat panel, electrophoretic display apparatus has been described in copending application Ser. No. 345,825 filed May 1, 1989 entitled Dual Anode Flat Panel Electrophoretic Display Apparatus for Frank J. DiSanto and Denis A. Krusos the inventors herein and assigned to Copytele Inc. the assignee herein. That application describes an electrophoretic display which has a grid cathode matrix arrangement consisting of a first plurality of parallel conductive lines insulated from a second plurality of parallel conductive lines which are transverse to said first plurality. Located with respect to the grid and cathode are first and second anode structures. The first anode is removed from the second with the second anode overlying the grid lines of the display and insulated therefrom. The second anode is biased to implement typical hold and erase modes independent of the first anode and the display structure as indicated in the application exhibits many advantages when compared with prior art displays.
Essentially the electrophoretic display (EPID) is well known and there exists many patents and articles in the prior art which describe operation and characteristics as well as describing methods of fabricating such displays. The following patents are illustrative of prior art devices and approaches. These patents issued in the name of Frank J. DiSanto and Denis A. Krusos, the inventors herein and are assigned to Copytele, Inc. the assignee herein. Apart from the patents to be described below there are of course many references which were cited in the prosecution of these patents as well as additional references which were cited in the background of these patents. Many of the techniques and methods of fabricating such displays are disclosed in the patents that follow, as well as in the additional references as indicated.
Essentially, one is referred to U.S. Pat. No. 4,655,897 issued on Apr. 7, 1987 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS. This patent describes a typical electrophoretic display apparatus utilizing an X-Y matrix consisting of grid and cathode lines which are insulated one from the other and which are associated with an anode electrode. The space between the grid and cathode lines and the anode electrode is filled with an electrophoretic dispersion. The patent describes techniques for making such displays as well as suitable dispersions and materials for use with such displays.
U.S. Pat. No. 4,732,830 issued on Mar. 22, 1988 entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS. This patent describes methods for making electrophoretic displays as well as describing display construction and operation.
U.S. Pat. No. 4,742,345 entitled ELECTROPHORETIC DISPLAY PANEL APPARATUS AND METHODS THEREFORE issued on May 3, 1988. This patent describes improved electrophoretic display panels exhibiting improved alignment and contrast as well as describing circuitry for use with such a display and methods for fabricating such a display.
U.S. Pat. No. 4,746,917 issued on May 24, 1988 entitled METHOD AND APPARATUS FOR OPERATING ELECTROPHORETIC DISPLAYS BETWEEN A DISPLAY AND A NON-DISPLAY MODE. This patent describes various biasing techniques for operating electrophoretic displays to provide writing, erasing as well as operating the display during a display and non-display mode.
U.S. Pat. No. 4,772,820 issued on Sept. 20, 1988 entitled MONOLITHIC FLAT PANEL DISPLAY APPARATUS. This patent describes methods and apparatus for fabricating flat panel displays employing electrophoretic principles as well as structures to enable such displays to be biased and driven by additional circuitry.
Thus as one can see an important consideration of the prior art is to provide an improved display with increased contrast, faster operating time, and more reliable performance. As indicated in the above noted copending application, a particularly disturbing problem which occurs in electrophoretic displays results in the appearance of a bright "flash" emanating from the display during the ERASE mode. This "flash" while occurring over a relatively short period is perfectly visible. The "flash" appears disturbing to many individuals who view the display and is caused by the following phenomenon. For proper operation of the electrophoretic display, the amount of pigment in the suspension is considerably greater than the pigment required to give a suitable background when the panel is in a HOLD condition. The term HOLD is well known in the art, and essentially the electrophoretic panel is placed in a HOLD condition prior to writing the display. In this manner the HOLD condition is achieved when the anode is at a high positive voltage, the grid is at a low voltage, and the cathode is at a high voltage.
Typically the anode would be at a voltage for example of 200 volts with the grid at a lower voltage as for example -12 volts with the cathode at a high voltage which would be +15 volts. With these voltages on the typical prior art electrophoretic display, the display is in the so-called HOLD condition. This HOLD condition is implemented prior to the WRITE mode during which mode new information is written into the display. The excess pigment during the HOLD condition is at the surface of the anode which is at the highest potential with respect to any other of the electrodes. To ERASE the display, the anode is made negative and all the pigment leaves the anode and is at the surface of the grid and cathode. During ERASE, the anode, for example, would be placed at -200 volts. Hence during the ERASE mode, all the pigment leaves the anode and is now at the surface of the grid and cathode accordingly. The cathode side of the flat display during the ERASE mode is considerably brighter than it is during the HOLD causing a bright "flash" to appear on the display even when the ERASE time is extremely short.
The "flash" occurs between frames and may exist every 20 to 30 milliseconds caused by the change in brightness between the HOLD and the ERASE mode. It is, of course, desirable to eliminate this bright "flash" so that the display appears more uniform and stable.
Hence the above noted copending application describes the addition of a second anode which is interposed between the original anode of the electrophoretic display and the cathode to grid matrix. The second anode aides in preventing this flash by allowing the pigment to be selectively controlled.
This present application describes methods for fabricating a dual anode display. The methods to be described shows a second anode or a additional anode which in one embodiment is designed to be parallel with the grid structure and in another embodiment is transverse or perpendicular to the grid structure. These methods include the selective deposition of different materials such as metals and insulators together with the use of selective etches in order to achieve the desired results.